The present invention relates to a power MOS FET and, more particularly, to such a MOS FET having a current sensing terminal.
A power MOS FET is usually constituted by a number of MOS FET cells which are connected in parallel to each other. It is frequently necessary to detect a current flowing through such a power MOS FET. Since such current may be increased, up to the order of 10 Amperes, it is not practical to measure such large current directly.
In order to solve this high current problem, a MOS FET having a current sensing terminal is disclosed in U.S. Pat. No. 4,553,084. In this patent, a current sensing element comprises a limited number of MOS FET cells connected in parallel with a power MOS FET. In such a MOS FET, having a current sensing terminal, the power MOS FET has a pair of source terminals. One of the source terminals is called a "Kelvin terminal" and the other to a "Miller terminal". A sense resistor having a value R is connected between the Miller terminal and the Kelvin terminal. A current I flowing through the power MOS FET can be defined by the following equation if the value R and a potential difference V across the sense resistor R are known: EQU V.apprxeq.IR/n
where n is a ratio number between MOS FET cells constituting the power MOS FET and MOS FET cells constituting the current sensing element.
Assuming that n=3000, R=30 ohms and I=10A, the potential difference V of 100 mV arises at both ends of the sense resistor R.
In the conventional MOS FET having a current sense terminal, the sense resistor is provided externally of the power MOS FET. Therefore, the sense resistor tends to be considerably influenced by external noise, causing the current detection to be inaccurate.
For a large external noise, it may be possible to detect the noise and to exclude it from a measured result. However, a circuit construction for performing these procedures may become very complicated and the sensing efficiency may be also degraded.